The present invention relates to heat motors and, more particularly, to motors which operate by the expansion of a contained material in response to an increase in the temperature of the material.
Heat motors containing a thermally expansible material, such as a wax, to extend a piston and thereby do work have long been known. Such motors have commonly been used in connection with vehicle thermostats to control the flow of cooling system liquid in the vehicle engine in response to the temperature of the cooling system liquid. In most cases, heat is imparted to the expansible material by the cooling system liquid flowing around the heat motor. It has also been known to actuate heat motors by wrapping an electric resistance heating element around a housing containing the thermally expansible material and to supply a current to the heating element in response to a certain predetermined condition. A significant problem with heat motors of the latter type is that heat must be transferred from the electric resistance heating element to the thermally expansible material through the cup containing the material and through insulation around the heating element. Thus, the response of the heat motor to the predetermined condition is slow.
It has also been proposed to place the electric heating element inside the wax to improve the response time of the heat motor. This arrangement has had the drawback that the heating element scorches the thermally expansible material and destroys its thermal expansion characteristics. In addition, expansion of the thermally expansible material during heating exerts strong forces on the electric heating element tending to push the heating element against a wall of the heat motor, damaging the element. There has also been a problem with leakage of the expansible material from the cup where electrical leads for the heating element have extended through the wall of the cup. Furthermore, the arrangement of the electric heating element in the cup has caused overstroking of the piston. The overstroking has been controlled by the use of a regulator connected in a circuit with the heating element. However, the requirement for an additional control device to be mounted and to take up space wherever a heat motor is used is unacceptable in many applications.
It has also been proposed to employ a thermistor having a positive temperature coefficient as a heating element for a heat motor in order to avoid overheating of the expansible material without using a separate regulator, since such thermistors are self-regulating in that their resistance increases as their temperature increases. The increased resistance reduces the current through the thermistor and thereby limits the temperature of the thermistor and the amount of heat it transfers to the expansible material. U.S. Pat. No. 3,696,611 to Noakes et al. discloses a thermistor provided as a sleeve around the outside of a thermal motor. U.S. Pat. No. 3,860,169 to Norman discloses a thermistor held in contact with the outside of a cup containing the expansible material. Thus, both of the aforesaid patents have attacked the problem of scorching the expansible material and overstroking the piston, but are still faced with the drawbacks of slow response time resulting from the heating element being positioned outside the cup containing the expansible material.
U.S. Pat. No. 4,685,651 to Nouvelle et al. discloses a thermostatic control device which, in one embodiment, includes a flat resistor printed on a flexible support positioned inside a capsule containing the expansible material, against the inner wall of the capsule. Electrical leads for the flat resistor extend out of the capsule through insulating rings of glass positioned in a plate welded to an end of the capsule.